Nematocysts of red-pigmented Cyanea capillata medusae and blue-pigmented Cyanea lamarckii medusae from the Swedish west coast were examined by light and scanning electron microscopy. In addition to homotrichous isorhizas and heterotrichous microbasic euryteles, a new heterotrichous microbasic rhopaloid nematocyst was identified. Nematocyst nomenclature is considered in the light of new SEM information; and changes in the definitions of isorhiza and anisorhiza nematocysts are suggested. In both species, the distribution of nematocysts on the marginal tentacles differed from that on the curtain-like oral-arms and exumbrella. On the tentacles the nematocysts occurred densely in batteries, but on the oral-arms they were diffusely scattered or appeared in batteries of varied size; generally they were dense only at the oral rim or nearby the rim. On the exumbrella nematocysts were loosely scattered or in small batteries. Nematocysts were more dense at the margin of the bell. Unexpectedly, nematocysts, particularly medium and large A-isorhizas, increased in size with increasing size of the medusae. Differences in size and distribution between morphologically identical nematocysts of the two Cyanea medusa species even of similar diameter were observed, thus strengthening the opinion that each is a valid species.